This invention relates to arc discharge lamps such as fluorescent lamps which operate at relatively high frequencies and in particular the present invention relates to circular fluorescent lamps having a centrally disposed ballast.
Because of the significant economic desire to conserve electrical energy, it has become increasingly desirable at the present time to increase the efficiency of electrical lighting systems. In particular, it is desirable to replace, to the extent possible, incandescent bulbs operating at efficacies of only approximately 15 lumens per watt with more efficient fluorescent lamp devices. Present fluorescent lamp devices operate at efficacies of approximately 40 lumens per watt or more. However, because of the nature of the arc discharge, special power supply problems exist for fluorescent lamps. The power supply circuits for such lamps are generally referred to in the art as ballasts. These ballasts, which are common in the fluorescent lamp arts, generally provide different power levels to the lamp because of the differences in lamp characteristics during startup and during the normal operation. In certain fluorescent lamps, the startup may be facilitated by the employment of filaments heated by a separate circuit in the ballast. Such lamps employ two conductors between each end of the lamp and the ballast. These are known as rapid start lamps. In other lamps, a single current supply is first used to heat the filaments and is then switched to power the discharge. The switching action is caused by a manually operated switch or an automatic, glow discharge, thermal switch, known as a starter. Such lamps employ one conductor between each end of the lamp and the ballast, and one conductor between each end of the lamp and the starting switch. These are known as switch start lamps. In a third type of lamp, starting is accomplished by providing a high voltage to initiate the discharge between electrodes disposed at either end of the lamp. Such lamps employ one conductor between each end of the lamp and the ballast. These are known as instant start lamps.
It has recently been determined that the weight and material requirements of the ballast can be significantly reduced if the lamp is operated at frequencies above 15,000 Hz. Such operation has also been found to promote increased lamp efficacy. However, it is also known that lamps operating at such high frequencies, that is, frequencies in excess of 15,000 Hz, can produce electromagnetic interference potentially capable of disturbing radio and television reception. If the fundamental frequency of an electronic inverting ballast lies below the AM broadcast band (535 kHz to 1,605 kHz), the most serious interference problem is caused by the magnetic field radiated by the lamp/ballast system. The electric field is less of an interference problem since AM radio receivers generally used in the home are designed to respond to the magnetic field component of an electromagnetic wave and are relatively insensitive to the electric field component. Magnetic field radiation is produced by electric currents flowing in conductors, and in particular for the applications intended here, magnetic field radiation is produced by the current flowing in the discharge lamp itself. The intensity of the radiated magnetic field is proportional to the current flowing in the circuit multiplied by the area of the current loop. This quantity is generally referred to as the magnetic moment.
The radiation of magnetic field interference is generally controlled in several ways. For example, a conductive shield could be placed around the offending current loop. Thus, it is easy to control electromagnetic interference emanating from the ballast itself simply by employing a conductive shield. However, it is significantly more difficult to provide proper shielding for the lamp itself because it is desirable to employ a material which possesses not only high electrical conductivity but also high light transmissivity. Another means of controlling electromagnetic interference is to filter the ballast output waveform to eliminate frequency components in the AM frequency band. While the fundamental frequency of most electronic ballasts is below 535 kHz, nonetheless, interference is caused by harmonics of the fundamental frequency which are generated by the ballast or lamp and radiated by the current loop within the lamp envelope. Moreover, it is generally true that high efficiency inverters generate output waveforms which include these undesirable harmonics. These interference producing harmonics may be filtered out of the ballast waveform before it is applied to the lamp, but such filters usually dissipate power, are physically large, and expensive.